Halo dynamics influences global accelerator performance: beam lifetimes, emittance growth, dynamic aperture, and collimation efficiency. Halo monitoring and control are also critical for the operation of high-power machines. For instance, in the high-luminosity upgrade of the LHC, the energy stored in the beam tails may reach several megajoules. Fast losses can result in superconducting magnet quenches, magnet damage, or even collimator deformation. The need arises to measure the beam halo and to remove it at controllable rates. In the Tevatron and in the LHC, halo population densities and diffusivities were measured with collimator scans by observing the time evolution of losses following small inward or outward collimator steps, under different experimental conditions: with single beams and in collision, and, in the case of the Tevatron, with a hollow electron lens acting on a subset of bunches. After the LHC resumes operations, it is planned to compare measured diffusivities with the known strength of transverse damper excitations. New proposals for nondestructive halo population density measurements are also briefly discussed.

This book introduces the physics and technology of the High-Luminosity Large Hadron Collider (LHC), highlighting the most recent modifications that shaped the final configuration, which is now in the advanced stages of its construction.This new High-Luminosity configuration of the LHC is the major accelerator project of this decade and will give new life to the LHC after its first fifteen years of operation, allowing for more precise measurements of the Higgs Boson and extending the mass limit reach for new particles.The LHC is such a highly optimized machine that upgrading it requires breakthroughs in many areas. Unsurprisingly, the High-Luminosity LHC required a long R&D period to bring into life an innovative accelerator magnet, based on Nb3Sn and capable of generating fields in the 11-12 T range, as well as many other new accelerator technologies such as superconducting compact RF crab cavities, advanced collimation concepts, a novel powering technology based on high temperature superconducting links, and others.The book is a self-consistent series of papers, which addresses all technology and design issues. Each paper can be read separately as well. The first few papers provide a summary of the whole project, the physics motivation, and the accelerator challenges. Altogether, this book brings the reader to the heart of the technologies that will also be key for the next generation of hadron colliders.This book is an essential reference for physicists and engineers in the field of hadron colliders and LHC related issues and can also be read by postgraduate students.

During a store, particles from the beam core continually diffuse outwards into the halo through a variety of mechanisms. Understanding the diffusion rate as a function of particle amplitude can help discover which processes are important to halo growth. A collimator can be used to measure the amplitude growth rate as a function of the particle amplitude. In this paper we present results of diffusion measurements performed at the Relativistic Heavy Ion Collider (RHIC) with fully stripped gold ions, deuterons, and protons. We compare these results with measurements from previous years, and simulations, and discuss any factors that relate to beam growth in RHIC.

Hollow electron lenses are considered as a possible means to improve the LHC beam collimation system, providing active control of halo diffusion rates and suppressing the population of transverse halos. After a very successful experience at the Tevatron, a conceptual design of a hollow e-lens optimized for the LHC was produced. Recent further studies have led to a mature preliminary technical design. In this paper, possible scenarios for the deployment of this technology at the LHC are elaborated in the context of the scheduled LHC long shutdowns until the full implementation of the HL-LHC upgrade in 2023. Possible setups of electron beam test stands at CERN and synergies with other relevant electron beam programmes are also discussed.